Method of degasifying steel and other metals



March 20, 1962 J. o. EDSTROM ETAL 3,026,195

METHOD OF DEGASIFYING STEEL AND OTHER METALS Filed March 25. 1958 F162FIG.3

Patented Mar. 20, 1952 [ice 3,026,195 METHDD F DEGASIFYING STEEL ANDOTHER METALS John Olaf Edstriim, Hantverkarbacken 38G, Sandviken,Sweden, and Gustaf Henrik Widmark, Enspannargatan 56,Stockholm-Vallingby, Sweden Filed Mar. 25, 1958, Ser. No. 723,744 Claimspriority, application Sweden Mar. 26, 1957 3 Claims. (Cl. 75-49) Thepresent invention relates to a method of degasifying a metal so as toremove dissolved gases, such as oxygen, hydrogen, nitrogen, carbonmonoxide, carbon dioxide, water vapour and the like, without reactionproducts of these gases remaining in the metals and without using pumpsfor the removal of the gases.

For many years metallurgists have tried to manufacture steel and metalproducts with the lowest possible content of oxygen, nitrogen andhydrogen. Under normal circumstances it is not possible to free steel orother metals of gaseous or non-metallic inclusions causing adeterioration of the mechanical and physical properties of the steel orother metals.

In steel, oxygen is found atomically dissolved and is very easilyremoved in the form of carbon monoxide according to the reaction Fromthis formula it becomes evident that it is possible to eliminate oxygenefiectively, if the carbon monoxide pressure can be reduced to lowvalues. In the reaction formulas given in this specification, thesubstances dissolved in steel are indicated by underlining theirchemical symbols. In order to eliminate oxygen according to the formulaa pressure lower than mm. Hg is required.

Hydrogen and nitrogen can be eliminated by processes according to theformulas H 1/2H2 and N )1/2N2 respectively.

The difficulties in eliminating hydrogen and nitrogen are caused by thefact that the equilibrium contents decrease proportionally to the squareroot of the value of the gas pressure. In order to avoid the formationof flakes, the hydrogen content should be lower than 0.0003%, whichcorresponds to an equilibrium pressure of 10 mm. Hg. For the effectiveelimination of oxygen, nitrogen or hydrogen from steel or other metals,of course, the highest possible vacuum should be attained.

The vacuum treatment can be applied through the melting of steel orother metals in induction melting furnaces which, however, are oflimited capacity and of complicated and expensive design. About 1940, amethod was found which allowed the vacuum treatment of steel in largequantities. According to this method, steel in liquid state is treatedin ladles or chills in a special chamber from which the gases can beevacuated by means of vacuum pumps. For 16 tons of steel in the ladles avacuum treatment of about minutes is required.

According to another method, a ladle which can be emptied through thebottom, is placed above a vacuum chamber in which a chill is provided.When the steel is being tapped down into the vacuum chamber, the moltenmetal stream is made to explode due to the gas pressure,

and the oxygen, hydrogen and nitrogen contents decrease very rapidly.During the tapping of the steel from the ladle, the vacuum is maintainedin the vacuum chamber by the exhaust of gas therefrom by suitable pumps.The pressure within the vacuum chamber is held at about 10 mm. Hg. Thismethod is most suitably used for casting large-size ingots.

Another degasifying method has been worked out which allows the vacuumtreatment of steel in large quantities in ladles. The ladle is placed ona platform above which a vacuum chamber, a so-called vacuum pipette,provided with pumping equipment is suspended on an overhead crane. Withthe only exception of a vertical, 1500 mm. long pipe leading into itsbottom, the vacuum chamber is completely sealed. The chamber is loweredsufiiciently so that the pipe is immersed in the steel, and the vacuumchamber is then evacuated. Thus, a certain steel quantity is sucked upinto the chamber and, after being degasified, is tapped down into theladle. This cycle is repeated several times, with the entire treatmentlasting about 30 minutes. While the oxygen as well as the hydrogencontent is reduced by one third, the nitrogen content is not subject toa considerable decrease, particularly when the nitrogen content is about0.004% at the beginning.

The abovementioned existing methods all seek to degasify the metals bymeans of a vacuum applied above the molten metals and created byevacuating the gas by means of pumps. At a high temperature and highvacuum, the gas volume evacuated is large in relation to the Weightevacuated, which necessarily requires a large capacity, and thereforeexpensive, pumping means. If, for example, steel is to be deoxidized at1600 C., a gas volume of about 700 mfi/ton steel is obtained While theoxygen content is reduced by 0.01% and the pressure is held at 10 mm.Hg. Besides, equally large volumes of hydrogen and nitrogen are to beeliminated at the same time.

The present invention relates to a method of eliminating these large gasvolumes in a rather compact state, namely in the form of solid orpossibly liquid reaction products to be obtained through the absorptionof the gases during a chemical or physical procedure.

A method embodying the present invention and usable for degasifyingmolten iron and steel will now be described. A representative example ofthe gas combination to be found above a molten steel surface undervacuum, is CO, 15% H and 5% N the normal gas quantity being 0.3 Nm. /tonsteel [=03 m. at STP (60 F., 29.92 Hg) per ton of steel].

From this gas mixture CO is eliminated, for example, according to thefollowing carbon deposition reaction At a temperature of, for instance,400 C. the equilibrium pressure is 10- mm. Hg, the reaction speed beingrather high. H can be eliminated, for example, according to the reactionformula for which reaction the equilibrium pressure is 10- mm. Hg at 400C. The quantities of CaO and Fe O consumed are small. The binding of Hrequires only about 1 kg. Fe O and 0.11 kg. CaO, and the binding of CO0.3 kg. CaO per ton of degasified steel. Owing to the fact that thematerials are regenerated without cost, no material is in realityconsumed.

N is eliminated, for example, through Al according to the reaction At atemperature as high as 1500 C., the equilibrium pressure is l0- mm. Hg,and at 400 C. it is only about 10- mm. Hg. 0.04 kg. Al is required perton of steel to be degassed. The expenditures for all reactants are thusinsignificant.

As shown by the reaction formulas and accompanying description givenbelow, Al can also be used for eliminating CO, H In thi case Al iseasily introduced into the evacuated chamber by injection thereof inmolten, dispersed or powdered state, whereby a large reaction surfaceand, consequently, a high reaction speed is attained. Mg, Ca, Si, andother metals, possibly also a mixture of them, are favourably used inthe same way. Beside metals in the pure state, it is so p ssi l to ualloys of metals and other compounds, as CaSi, CaAl, FeTi, AlTi, MgAl,and the like.

Generally, the elimination of gas can be achieved according to aplurality of reactions. As explained above, CO can be eliminated throughthe reaction with carbonate forming oxides (CaO, BaO, SrO, Na O, K 0,MgO, MnO, and the like) with the precipitation of free C in the presenceof catalysts (for example iron or some iron compound), or by thereaction with oxides which are easy to deoxidize (possible superoxides)and carbonate formers.

Principal formulas are 2C0 +MeO- MeCO +C 'Co-i- MeO+M O MeCO +M O CO isalso bound according to the reaction Me+CO MeO-|C for example with themetals Al, Si, Ca, Mg, Ba, Sr, Na, K, Fe, or Z, or by reactions withcarbides according to the typical formula for example with Al-, Ca-,Mg-, Fe-, Cr-, Ti-, Zr-, or Si-carbides. Another principal formula forbinding CO is As further examples, Me can be replaced by Ti, V, Cr, Zr,Si, Mo, B and other element with high atfinity to oxygen and carbon. H;can be eliminated according to the method exemplified above, i.e.through oxidation with an oxide which easy to deoxidize and theabsorption of water formed as an hydroxide. Normally, water is also tobe bound through usual absorption or adsorption means, such as CaClMgClO silica gel, H 50 A1 0 and the like. Hydrogen can also be bound ashydride according to the formula in which Me, may be replaced forexample, by Ca, Ba, Sr, Na, Li, Al, and the like. It is also possible toburn (possibly catalytically) H and CO to H 0 and CO through theinjection of oxygen gas whereby H 0 and CO are absorbed according to theabove method or alternatively condensed by cooling. The reactions areCO+ /2O +MeO- MeCO H /2O MeO Me(OH) N can be eliminated from a gaseousatmosphere through the reaction with metals, for example, Al, Ti, Zr,Ta, Mg, Cr, Mn, or Fe, or with silicon or boron, with the formation ofnitride.

2Me+N 2MeN Effects are also produced by reactions with carbides (forexample CaO A1 0 and others), for instance, according to the formulasMeC-FN MeCN Cir N can also be burned catalytically or oxidized in adifierent way, whereafter the oxidation products are, for example,condensed by cooling.

Also gases developed from metals other than CO, H N as for example, 0 COH O, halogens, sulphur compounds, phosphorous compounds, and the likecan be absorbed or adsorbed in a Way corresponding to the methoddescribed above.

Absorption methods can also be used for a selective elimination of gasesusually found above the surface of metals. It is, for instance, possibleto eliminate two of the gas components and to maintain only one gas, forexample, N in contact with the surface of the metal.

Several examples of apparatus suitable for the practice of the methodsembodying the present invention are hereinafter described in detail withreference to the accompanying drawing, wherein:

FIGS. 1 to 6, inclusive, are vertical sectional views of the respectiveapparatus.

In the several views of the drawing, the same reference numerals areused to identify the corresponding parts of the respective apparatus.

The absorption medium can be injected into the vacuum chamber either inportions which are fractions of the total amount required, or the lattercan be injected all at one time. Further, the absorption medium can beintroduced before the operation is started, or it may be added duringoperation. The vacuum obtained can be varied through varying thetemperature or quantity of the reactants, so that the degasificationintensity is thus easily controlled. This is especially easily done withthe apparatus according to FIG. 5 which is hereinafter described indetail.

FIG. 1 shows the ladle 1 with steel 2 therein placed in a vacuum chamber3 which completely surrounds the ladle 1 and is provided with a coverand a paclc'ng 5. The absorption medium is placed in a gas pervioussection 6 of the cover. The chamber 3 is provided with a control valve7. Since the absorption medium reacts with the gases within sealedchamber 3 to form liquid or solid reaction products, a vacuum isproduced in chamber 3.

FIG. 2 shows a ladle 1 with steel 2 therein provided with a cover 8sealed with the packing 5 so that the space Within cover 8 forms avacuum chamber. The absorption medium is placed in the section 6 of thecover. The cover 8 is provided with a control valve 7.

FIG. 3 shows the tapping of steel 2 from the ladle 1 provided with atapping valve 9 arranged for tapping through the bottom of ladle downinto a chill 10 placed in a vacuum chamber 11. A packing 13 is providedbetween the ladle bottom and the chamber 11. The absorption medium isplaced in a section 12 in chamber 11 or injected into the latter bymeans of the sprayer 14. The vacuum chamber 11 is provided with acontrol valve 7.

FIG. 4 shows the steel 2 sucked up from the ladle 1 into a vacuumpipette 15 through a depending pipe 16. The vacuum pipette can be liftedor lowered by means of an overhead crane, for which purpose the pipetteis provided with an eye 17 for the hook of the crane. The absorptionmedium is placed in the section 18 within the pipette, and the latter isequipped with a control valve 7. The vacuum required for raising steelfrom the ladle 1 into the pipette 15 can be brought about possiblyseveral times, through the absorption procedure which creates a vacuumin pipette 15 above the surface of the steel therein.

FIG. 5 shows a ladle 1 with steel 2 provided with a sealing cap 19 andwith absorption medium being placed in a separate section 20 of the capwhich is connected to the ladle 1 by means of pipes 21. Fractionatedabsorption can be performed, for instance, in the order CO, H; and NWhen the reaction has started, the gas is sucked from the steel to theabsorption medium by the relatively low pressure created in section 20.The absorption section 20 is provided with a control valve 22.

FIG. 6 shows a vacuum chamber defining cover 8 with a sealing packing 5placed above an induction furnace 23 with steel 2 in the latter. Theabsorption medium is placed in the gas pervious section 6 within thevacuum chamber defined by cover '8, and the latter is provided with acontrol valve 7.

While the above examples refer to molten steel, the method ofdegasifying all other types of molten metals is generally similarthereto.

Several of the reactions for degasitying molten metals, as proposed inthis description, were previously performed in a metal bath, whichcaused the solidified metals to contain non-metallic inclusions as, forinstance, oxidic slag. According to the method described, the oxygen,for example, is instead sucked up from the molten metal bath and allowedto form oxides at a place spaced from the metal bath. Thus, thesolidified metals are free from slags and non-metallic inclusions ingeneral.

What we claim is:

1. A method of eliminating at least one of the gaseous componentsdissolved in molten steel, comprising the steps of placing the moltensteel in a closed vessel having a volume greater than that of the moltensteel so as to provide a space above the free surface of the moltensteel and disposing in said space remote from said free surface at leastone substance in an amount of 0.04 to 1.1 kg./ ton of molten steel andselected from the group consisting of metals and metal alloys, oxidesand carbides capable of selectively reacting With only certain of saidgaseous components of the molten steel to produce nongaseous reactionproducts, thereby to create a vacuum of at least 10 mm. of Hg in saidspace, and evacuating the remainder of said gaseous substances bymechanical means from said space.

2. A method of eliminating at least one of the gaseous componentsdissolved in a metal, comprising the steps of placing the metal in aclosed vessel having a volume greater than that of the metal so as toprovide a space above the free surface of the metal, disposing in saidspace remote from said free surface at least one substance capable ofreacting selectively with only certain of said gaseous components of themetal to produce nongaseous reaction products, thereby to create avacuum of at least 10 mm. Hg in said space for promoting the furtherrelease of the gaseous components of the metal into said space forcontinuing the reaction with said substance, and evacuating theremainder of said gaseous substances by mechanical means from saidspace.

3. A method of eliminating at least one of the gaseous componentsdissolved in a metal, comprising the steps of placing the metal in aclosed vessel having a volume greater than that of the metal so as toprovide a space above the upper surface of the metal, and disposing insaid space above the upper surface of the metal at least one substancecapable of reacting selectively with only certain of said gaseouscomponents of the metal to produce non-gaseous reaction products,thereby to create a vacuum of at least 10 mm. Hg in said space forpromoting the further release of the gaseous components of the metalinto said space for continuing the reaction with said substance, andevacuating the remainder of said gaseous substances from said space bymechanical means.

References Cited in the file of this patent UNITED STATES PATENTS127,953 Bennett June 18, 1872 1,815,691 Wilson July 21, 1931 1,921,060Williams Aug. 8, 1933 2,144,200 Rohn et a1 Jan. 17, 1939 2,253,421 DeMare Aug. 19, 1941 2,452,665 Kroll et al. Nov. 2, 1948 2,776,886 Kellyet al. Jan. 8, 1957 2,837,790 Rozian June 10, 1958 FOREIGN PATENTS569,699 Germany Feb. 6, 1933 OTHER REFERENCES Gas Free Metals, TheMetals Research StaflF, National Research Corporation, Cambridge 42,Mass, September 1947, page 1 relied on.

1. A METHOD OF ELIMINATING AT LEAST ONE OF THE GASEOUS COMPONENTSDISSOLVED IN MOLTEN STEEL, COMPRISING THE STEPS OF PLACING THE MOLTENSTEEL IN A CLOSED VESSEL HAVING A VOLUME GREATER THAN THAT OF THE MOLTENSTEEL SO AS TO PROVIDE A SPACE ABOVE THE FREE SURFACE OF THE MOLTENSTEEL AND DISPOSING IN SAID SPACE REMOTE FROM SAID FREE SURFACE AT LEASTONE SUBSTANCE IN AN AMOUNT OF 0.04 TO 1.1 KG/TON OF MOLTEN STEEL ANDSELECTED FROM THE GROUP CON-